The invention relates to the field of photonic crystal waveguides, and in particular to a monolithically integrated waveguide structure that confines and guides light emitted from a laser or LED light source mounted on the backside of a silicon (Si) wafer, without power loss due to Si materials absorption in the wafer.
Photonic crystal waveguides have been demonstrated in cylindrical geometry fibers and recently in planar waveguides, by employing the principal of omnidirectional reflection for wavelengths of light whose optical states lie fully within a photonic bandgap, as confined by the light-line of the propagating medium.
FIG. 1A shows the photonic band diagram of a one-dimensional (1-D) periodic photonic crystal, and its comparison in FIG. 1B with the angular reflectivity spectra of Bragg Gratings (Reflectors)—the historically popular name for 1-D photonic crystals. A photonic band diagram plots the allowed propagation constant β values for different (angular) frequencies ω of light. These propagation constant values correspond to different angles of the light wavevector within the Bragg reflector structure. The (Fresnel) reflectivity spectrum of Bragg gratings have been studied extensively; the reflectivity stopband has been understood to be an interferometric effect based on the two refractive index materials comprising the Bragg grating, n1 and n2, having quarter-wavelength film thicknesses which are normalized with respect to refractive index: t1=λ/4/n1, t2=λ/4/n2. The combination of one t1 and one t2 layer are referred to as a Bragg pair of the grating.
The development of photonic crystal theory in the last fifteen years has arisen from the observation that the form of the Helmholtz equation for propagating modes of light is identical, to the form of the Schrodinger equation for propagating electron states. Analogous to the electron's conduction band states, valence band states, electronic bandgap and defect states within the bandgap, a periodic variation in refractive index modifies propagating modes of light to exist in either (1) low dielectric states (electric field intensity concentrated within the n1 Bragg pair regions), (2) high dielectric states (electric field intensity concentrated within the n2 Bragg pair regions), (3) a prohibited range of light frequencies referred to as the optical or photonic bandgap, and (4) defect layers of material that localize electric field distributions for light frequencies with propagation constant values lying within the photonic bandgap.
The photonic bandgap was immediately recognized to be the reflectivity stopband of Bragg reflectors. FIG. 1B shows how the reflectivity of a Bragg grating, for a given angle, can now be more generally understood as a straight line trajectory, with a given slope, on the photonic band diagram in FIG. 1A. This more generalized understanding of the Bragg interference phenomena shortly gave rise to a very important conclusion: a Bragg grating could reflect light incident from all angles in air, without the requirement that that there be a complete photonic bandgap, at the wavelength of interest.
Wavelengths of light incident from air onto the Bragg grating, or equivalently, the 1-D photonic crystal, will transmit into the structure only if there exist propagating modes within the light-line. The light-line is a line whose slope corresponds to the speed of light divided by the refractive index of the incident medium—in this case, air with n=1.0. For wavelengths at which there exists only the photonic bandgap, within the light-line, transmission into the structure will be prohibited. Hence an omni-directional reflector can be built using Bragg reflector materials n1 and n2, which otherwise do not possess a complete photonic bandgap.
An omni-directional reflector could be folded about itself to contain an air gap, or guiding defect layer, thus creating a coaxial structure which could trap light within the air gap and guide it along the axial direction. This new type of waveguide follows light propagation physics that differs from the total internal reflection based physics of conventional fiber optics and planar waveguides.